1. Field of the Invention
Embodiments of the invention relates to a display device, and more particularly, to a backlight unit and a display device having the same. Although embodiments of the invention are suitable for a wide scope of applications, they are particularly suitable for providing a backlight unit for a large-sized liquid crystal display device.
2. Discussion of the Related Art
A cathode ray tube (“CRT”) has been widely used for monitors of a television, a measuring machine and an information terminal. However, a CRT is bulky and heavy. Thus, various display devices, for example, a liquid crystal display (“LCD”) device using an electric field optical effect, a plasma display panel (“PDP”) using a gas discharge and an electroluminescence display (“ELD”) device using an electric field luminous effect, have been studied to substitute the CRT. Among the various display devices, LCD devices have been most actively studied. LCD devices have advantageous characteristics, such as compact size, lightness and low power consumption, and are suitable as flat display devices, for example, monitors for spacecrafts, aircrafts, notebook computers, laptop computers, desktop computers and large-sized display devices. Thus, the demand for the LCD devices is in the continuous increase.
In general, LCD devices control light transmittance from ambient light, to thereby display images, and requires a separate light source, such as a backlight unit. A backlight unit used as the light source of an LCD device commonly has a cylindrical light-emitting lamp, and the backlight unit is classified into a direct type and an edge type based on the arrangement of the light-emitting lamps.
In the edge type backlight unit, a lamp unit is provided at one side of a light-guiding plate, and the lamp unit has one lamp for emitting light. A lamp holder is inserted into both ends of the lamp to protect the lamp, and a reflective sheet, which has one side inserted to the light-guiding plate and surrounding the circumference of the lamp, reflects the light emitted from the lamp to the light-guiding plate. The edge type backlight unit is generally applied to relatively small sized LCD devices for the monitors of the laptop computer and the desktop computer. The edge type backlight unit is advantageous in that it has great uniformity of light, long life span, and thin profile of the LCD device. However, the edge type backlight unit has the low uniformity of light when the edge type backlight unit is used as the light source of the large-sized LCD device.
With trend of the large-sized LCD device of 20-inch or more, the direct type backlight unit is actively developed. In the direct type backlight unit, a plurality of lamps are formed along one line on a lower surface of a light-diffusion sheet, whereby the entire surface of the LCD panel is directly illuminated with the light emitted from the lamps. The direct type backlight unit has greater light efficiency as compared with the edge type backlight unit. Thus, the direct type backlight unit is used for the large-sized LCD device requiring high luminance, and the LCD device having the direct type backlight unit is generally used for the large-sized monitor or television. However, with the direct type backlight unit, it is difficult to obtain the thin profile of the LCD device since the plurality of lamps are directly provided on the light-diffusion sheet.
FIG. 1 is a cross-sectional view illustrating a backlight unit according to the related art, FIG. 2A is an exploded perspective view illustrating a backlight unit according to the related art, and FIG. 2B is an exploded cross-sectional view illustrating a backlight unit according to the related art. FIGS. 2A and 2B show one example of an edge type backlight unit to convert a line-type light source into a surface-type light source. In FIG. 1, the related art backlight unit includes a main supporter 1 which supports respective components, and a lower cover 3 which protects a lateral side of the main supporter 1 and a lower surface adjacent to the lateral side of the main supporter 1. The backlight unit also includes a light-guiding plate 5 which guides light emitted from a lamp 10 to an LCD panel (not shown). The lamp 10 is provide at an incidence surface corresponding to one side of the light-guiding plate 5, and a lamp assembly accommodates the lamp 10. The backlight further includes lower and upper light-diffusion sheets 6 and 9, which are provided on an upper surface of the light-guiding plate 5 to diffuse the light guided by the light-guiding plate 5, and lower and upper prism sheets 7 and 8, which concentrate the light diffused by the lower and upper light-diffusion sheets 6 and 9 and transmit the concentrated light to the LCD panel (not shown). The lamp 10 is a cold cathode fluorescent lamp (“CCFL”), which generates visible rays according to a high voltage applied from an inverter.
As shown in FIGS. 2A and 2B, a high-voltage lamp wire 13a and a low-voltage lamp wire 13b are connected with a connector 16, and are also respectively inserted into a high-voltage lamp holder 12a and a low-voltage lamp holder 12b. After the lamp wires 13a and 13b are soldered on the high-voltage and low-voltage lamp holders 12a and 12b, the lamp holders 12a and 12b are put into a lamp housing. Thus, the soldering portions of the lamp are covered with the lamp holders 12a and 12b, thereby completing the lamp assembly.
Then, the lamp assembly is assembled into the main supporter 1, and the lower cover 3 is provided to the incidence side of the main supporter 1 to protect the lamp assembly from the external force. After that, the reflective sheet 4 is provided on an inner bottom of the main supporter 1, the light-guiding plate 5 is provided around the inner gap of the lamp housing 15 in consideration for the gap size and the flatness of the lamp housing 15.
The lower light-diffusion sheet 6, the lower prism sheet 7, and the upper prism sheet 8 and the upper light-diffusion sheet 9 are provided above the light-guiding plate 5 in sequence. In the above-mentioned backlight unit according to the related art, power is applied to the lamp through the use of connector connected with a power supply, such that the lamp emits light by a glow discharge. As light emitted from the lamp is incident on the incidence surface of the light-guiding plate 5, light then is reflected on and diffused by printing dots 17 provided on the lower surface of the light-guiding plate 5. Then, the diffused light is concentrated to a vertical direction through the use of lower and upper prism sheets 8 and 7, and is then slantingly diffused through the light-diffusion sheets 6 and 9. The reflective sheet 4 reflects the light, which leaks to the rear surface without being reflected and diffused on the printing dots 17, to the upper direction.
Then, the rear of the LCD panel is illuminated with the predetermined amount of light through the upper light-diffusion sheet 9. Since the main purpose of the upper light-diffusion sheet 9 is to protect the upper prism sheet 8, the upper light-diffusion sheet 9 may be referred to as a protection film. If using the backlight unit for a monitor, the light-diffusion sheet and a double brightness enhanced film (“DBEF”) are integrated into one, to thereby obtain a wide viewing angle.
The edge type backlight unit according to the related art has the following disadvantages. The related art edge type backlight unit is generally applied to the small-sized LCD device, for example, the monitors for the laptop and desktop computers. If the edge type backlight unit is applied to the large-sized LCD device, it is difficult to obtain the uniform brightness on the entire surface of the LCD panel. In addition, since the lamp is provided at one side of the backlight unit and the light emitted from the lamp is required to pass through the light-guiding plate, it is difficult to obtain the high brightness.
The edge type backlight unit according to the related art is provided with the lower and upper light-diffusion sheets and the lower and upper prism sheets above the light-guiding plate. Thus, the edge type backlight unit according to the related art is increased in thickness. Further, if integrating the upper light-diffusion sheet with the DBEF, the price competition is lowered due to the expensive DBEF.